The study will entail a genetic and cytogenetic analysis of both a euchromatic and a heterochromatic segment of the Drosophila melanogaster genome. The analysis of the euchromatic segment in the proximal portion of chromosome 3R involves the saturation, in genetic terms, of a cytologically well defined section of the genome. It is hoped that this process will further illuminate the one gene one band equivalence which has been found elsewhere in the genome and will clarify, due to the informational content of this segment, the exceptions to the above rule of thumb. The genetic and cytogenetic analysis of the heterochromatic Y chromosome should reveal new information on the functional organization of this less well understood segment of the eucaryotic genome. The new techniques of fluorescent staining and chromosome banding as well as in situ hybridization of specific DNA sequences to the metaphase chromosomes allow the possibility of correlating genetic, cytogenetic and satellite DNA maps of the Y chromosome and contrasting the functional organization of hetero- as opposed to euchromatin.